The present invention relates generally to a process for increasing the strength of polymers and articles produced thereby. In particular, the present invention is directed to bone healing devices made from higher strength bioabsorbable polymeric materials.
Traditionally, certain types of bone fractures have required the use of particular bone fixation devices or supporting members in order to promote proper healing of the bone. As used herein, a bone fixation device is any device used to heal bones and allied tissues such as cartilage, ligaments, etc. Such bone healing devices include plates, bone screws, wires, pins, staples, table ties, clips and the like. Generally, the devices are used in order to bring and maintain the fractured surfaces of a bone into close contact by applying a compressive force to the bone. Compression must be applied to the bone until there is sufficient healing for the bone to support normal loads.
One particular bone fixation device that has been found to be very useful in healing bone fractures is the Herbert bone screw which is disclosed in U.S. Pat. No. 4,175,555. The Herbert bone screw is a screw that includes a shaft having threaded portions at each end. Although like-handed, the pitch of the threaded portion at the leading end of the shaft is slightly greater than the pitch of the threaded portion at the trailing end of the shaft. When used to heal a bone fracture, this differential pitch causes the leading edge of the screw to advance axially into the remote fragment to a slightly greater extent than it does relative to the near fragments. This action puts the screw in tension and hence the fracture faces of the bone under compression. Further, the Herbert screw does not include a screw head, thus allowing the entire screw to be embedded within the bone without any protrusions.
The Herbert differential pitch bone screw has been used successfully in many different types of bone fractures and has gained growing acceptance by the medical industry. Specifically, the Herbert screw is recommended for use in fractures of the basal joint of the thumb, fractures of the head of the radius, treatment of delayed and non-union scaphoid fractures or their malunion, treatment of trans-scaphoid perilunate dislocation, internal fixation of capitellar fractures, and treatment of osteochondral fractures at various cites in the body. It is used as an alternative to wires, pins or small conventional screws.
Herbert bone screws, similar to other conventional bone fixation devices, are typically made from metals such as stainless steel or titanium alloys. However, although metal bone devices have achieved relatively high degrees of success in repairing bone fractures, these devices have some undesirable features. For instance, these metallic devices must be removed after the bone fracture has healed but before bone resorption, loosening, corrosion or infection. Removing the devices requires a second surgical procedure resulting in additional trauma to the patient as well as increased medical costs.
Another disadvantage to using bone fixation devices made from metallic substances is that the metal typically has a high modulus, is stiff and is mechanically incompatible with the bone. Use of such devices with high stiffness has been identified as a cause of stress shielding. Basically, when stress shielding occurs, the bone fixation device absorbs most of the stress applied to the bone and insulates the surrounding bone from normal loading. Stress shielding can be the cause of significant bone resorption, with consequent reduction in the strength of the bone in the region of the healed fracture. The bone thus becomes susceptible to refracture after the device is removed.
Bone fixation devices made from metals can also cause other problems including the generation of toxic metallic ions and the potential for infection. Consequently, a need exists for a bone fixation device not made from a metallic substance or alloy and which does not have to be removed from the bone. One possible solution or alternative to metallic bone healing devices is the use of polymeric materials and specifically the use of bioabsorbable polymeric materials. As used herein, a bioabsorbable material is defined as a material capable of being broken down, degraded, and ultimately absorbed by the body. For instance, bioabsorbable polymeric materials are typically degraded by the human body through a series of hydrolysis reactions which break down the polymer chains into smaller chemical units. Typically, the polymer is reduced mostly to water and carbon dioxide and absorbed into the digestive system. In the past, others have attempted to incorporate polymeric and bioabsorbable materials into bone fixation devices.
For example, U.S. Pat. Nos. 4,539,981 and 4,550,449 both to Tunc disclose a process for producing an absorbable bone fixation device. Specifically, the bone fixation device is made from a high molecular weight polylactide polymer having a low unreacted monomer content. In particular, the monomer content must be below about 2%. The process and conditions of forming the polylactide must be very carefully controlled in order to produce a polymer with sufficient strength. In fact, the reaction time in order to produce a polymer having desired characteristics is between 50 and 120 hours.
U.S. Pat. No. 4,756,307 to Crowninshield discloses a nail device to be implanted into a bone that includes a plurality of segments fastened together by a resorbable bioadhesive such that the plurality of segments are gradually disassociated after the nail device has been implanted. Specifically, the segments are joined by a "bioadhesive" such as polylactic acid, polyglycolic acid, copolymers or blends of these two, or a protein based adhesive. Once implanted, the nail device exhibits high strength characteristics and is resistent to bending and rotation. After the bioadhesive is resorbed, the plurality of segments will freely disassociate to restrict bone bending and torsion to a lesser extent than previously restricted with the solid rod. The nail device is particularly suited to fractures in the femur bone.
A resorbable compressing screw and method are disclosed in U.S. Pat. No. 4,776,329 to Treharne. In particular, Treharne is directed to hip screws in which the conventional metallic screw is replaced in whole or in part by a resorbable material. Preferably, the resorbable material is poly(DL-lactide) with a molecular weight of 40,000 to 160,000. The compression screw is formed by injection molding.
U.S. Pat. No. 4,858,603 to Clemow et al. discloses a bone pin made with a tapered polymeric portion and a cutting device secured to the smaller end of the polymeric portion. The cutting device can be used as a drill point for drilling through the bone. The polymeric portion can be made from a polymer which is absorbable in an animal body.
In U.S. Pat. No. 4,338,926 to Kummer et al., a bone prosthesis for use in healing a bone fracture is disclosed comprising a strong, rigid non-absorbable structural member and a biologically absorbable element held in use under compression against the structural member when the prosthesis is secured to the bone. After implantation, the biologically absorbable element is gradually absorbed by the body shifting stress transmission from the prosthesis to the healing bone.
A fixation screw for securing a bone graft of a tendon section emplaced in a ligament tunnel is disclosed in U.S. Pat. No. 5,062,843 to Mahony. The fixation screw can be formed from a biocompatible plastic or bioabsorbable material that is soft for preventing the threads of the screw from cutting into and damaging the bone grafts. Preferably, the material is made from ultrahigh molecular weight polyethylene.
A process for producing ultrahigh molecular weight ]polyethylene products including orthopedic prosthetic implants is disclosed in U.S. Pat. No. 5,030,402 to Zachariades. However, polyethylene is not considered a bioabsorbable polymer. The process includes compressing ultrahigh molecular weight polyethylene between a pair of molding plates to produce the shape of the final product. A perimeter access zone is included for allowing excess polymer in the mold cavity to deform past the mold cavity during its compression. The mold may be circular, rectangular or any other shape.
Other bone fixation devices are disclosed in U.S. Pat. No. 4,858,601 to Glisson which is directed to an adjustable compression bone screw; U.S. Pat. No. 4,723,541 to Reese which is directed to a bone screw and method; U.S. Pat. No. 4,463,753 to Gustilo which is directed to a compression bone screw; and U.S. Pat. No. Re. 33,348 to Lower which is directed to a bone screw.
Although the prior art shows the use of various bone fixation devices made in whole or in part from bioabsorbable materials, the prior art still has many deficiencies and drawbacks. Ideally, the materials used to make the bone fixation devices would have the initial biocompatibility, strength and ductility of stainless steel, would retain these properties for several weeks or months; and then undergo benign and complete biodegradation, absorption, and/or excretion. However, in the past such bioabsorbable materials have not exhibited enough strength or have retained enough strength for a required period of time.
All published work from 1980 to 1988 regarding mechanical properties of biodegradable polymers and composites proposed for use in internal fixation in place of stainless steel was reviewed and critiqued in an article entitled "Mechanical Properties of Biodegradable Polymers and Composites Proposed for Internal Fixation of Bone" authored by Daniels et al. In conclusion, Daniels et al. stated that completely biodegradable materials described to date do not meet all the requirements for replacing stainless steel devices including especially stiffness and ductility. In summary, the prior art devices have been only moderately successful. The bioabsorbable materials disclosed in the past have only been used in particular clinical applications using specific methods of fixation that are designed to be compatible with the physical characteristics of the material.
The present invention recognizes and addresses the above-described deficiencies and drawbacks of the prior art. In particular, the present invention is directed to a process for enhancing the physical characteristics of bioabsorbable materials and of all polymers in general. Specifically, the process of the present invention increases the mechanical properties of polymers by orienting the molecular chains contained within the material.